In a multi-type air conditioner employed in air conditioning of a building, etc., the length of refrigerant pipes that connect a plurality of indoor units and outdoor units becomes long. Because of this, during the operation of the air conditioner, lubricant that has flowed out together with refrigerant from a compressor to a refrigerant circuit side tends to remain at the refrigerant circuit side, and a situation tends to occur in which the amount of lubricant at the compressor side becomes insufficient. Therefore, in such an air conditioner, in order to recover the lubricant that has flowed out from the compressor to the refrigerant circuit side back into the compressor, an oil-return operation is performed at predetermined timing by counting up the cumulative operating time of the air conditioner or by calculating the amount of lubricant that has flowed out from the compressor and by detecting that the amount has reached a predetermined amount (for example, see Patent Literatures 1 and 2).
This oil-return operation is generally performed by setting a refrigerating cycle to a cooling cycle, by setting a cooling expansion valve on an indoor unit side to open, and by increasing the circulated amount and flow speed of refrigerant by increasing the rotational speed of the compressor. By doing so, lubricant remaining in the refrigerant circuit, such as a heat exchanger, refrigerant pipes, etc. is recovered back into the compressor along with the refrigerant.
On the other hand, when an operation which is under high pressure to begin with is switched to the oil-return operation, the high pressure sometimes abruptly increases because the amount of circulated refrigerant increases due to an increase in the rotational speed of the compressor. When the high pressure undergoes a transient abnormal increase, a high-pressure pressure sensor for protection against high pressure is activated, and the compressor may come to an abnormal halt due to high-pressure shutdown. Therefore, Patent Literature 3 discloses a technique in which, when conditions for oil recovery are met, an abnormal increase in the high pressure is suppressed in accordance with the operating conditions at that time by decreasing the amount of refrigerant circulated by bypassing discharged gas or by increasing the condensation capacity with an outdoor fan.